This invention relates to a method of improving the uniformity and quality of the base insulating coating on silicon-iron steel. More particularly, this invention relates to a method of improving base coating formation on silicon steel and the magnetic quality thereof by controlling the tension of winding into coil form.
Silicon steel or silicon-iron steel is useful for its electrical and magnetic properties and may be as oriented or non-oriented steels. In the production of such steels, an annealing separator coating may be used to improve the magnetic properties and prevent sticking of coil laps during heat treatment. Annealing separator coatings are particularly useful with grain-oriented silicon steels.
Grain-oriented silicon steel produced in strip form is useful for various electrical applications, such as laminates used in transformer cores and the like. The desired grain orientation, such as cube-on-face or cube-on-edge, is produced during a final high temperature annealing operation. For such purposes, the silicon steel is hot rolled to form a hot-rolled band, pickled, and then cold rolled to final gauge by a series of cold-rolling operations with intermediate anneals, decarburized, coated with an annealing separator coating, and then final high temperature annealed in coil form to achieve the desired secondary recrystallization and grain orientation texture. The secondary recrystallization is achieved by inhibiting primary grain growth during stages of the annealing operation wherein this occurs. This is conventionally achieved by providing primary grain growth inhibitors, such as boron, manganese sulfides, and aluminum sulfides.
Prior to final texture annealing, the steel is conventionally coated with an annealing separator coating, such as magnesium oxide. Conventionally, such a coating may be applied by slurry coating, roller coating, dipping, or electrolytically coating the surfaces of the strip. The strip is then typically wrapped in coil form for final texture annealing at temperatures on the order of about 2200.degree. F. (1404.degree. C.). The annealing separator coating prevents the convolutions of the coil from bonding together during the high temperature annealing treatment, and in addition reacts with the silica present on the surface of the steel strip to form a strong forsterite or glass-insulating film. The coating also improves the magnetic properties of the silicon steel by removing sulfur after secondary recrystallization has taken place during the final high temperature texture annealing.
Moisture present in the annealing separator coating, such as in the form of magnesium hydroxide in the magnesium oxide coating, is liberated during initial stages of final texture annealing to cause transient oxidation of the steel surface as the iron reacts therewith to form iron oxides. Such excess moisture results in irregular coating of the steel having bare, uncoated areas and poor base coating development and deposits of reduced iron oxide on the strip surface. This poor surface quality impairs the magnetic performance of the steel for final electrical product applications. After final texture annealing of the silicon steel with the coating thereon, the steel strip is typically "scrubbed" to remove the annealing separator coating.
As used herein, the "performance", or, more specifically, "scrub performance" refers to the surface quality of the forsterite insulating coating, i.e., base glass coating, wherein poor surface quality is characterized by uncoated areas and iron oxide deposits.
Attempts have been made by others to improve the annealing separator coating in order to result in better scrub performance. Furthermore, in a co-pending U.S. application, Ser. No. 607,889, filed May 7, 1984 now U.S. Pat. No. 4,582,547 and assigned to the common Assignee of the present application, there is disclosed a method for producing an annealing separator coating on silicon steel prior to final texture annealing to improve the coating uniformity and prevent oxidation of the steel surface during annealing by applying to the steel a coating such as magnesium oxide having as an addition an inert high termperature refractory annealing separator agent in the form of particles.
Accordingly, it is a primary object of the present invention to provide a method for improving the scrub performance of the grain-oriented silicon steel without modifying the annealing separator coating while avoiding the adverse effects of liberated water.
A further object is to substantially eliminate the iron oxide deposits on the silicon steel surface resulting from excess moisture between the coil laps.
It is also an object to improve the base coating development to provide better uniformity and quality of the coating.
A further object is to improve the core losses of silicon-iron strip and particularly cube-on-edge oriented silicon steel.
These and other objects of the invention, as well as a more complete understanding thereof, may be obtained from the following description and specific examples.